Novel yellow azo dyes and dichroic liquid crystal compositions made therewith

ABSTRACT

Novel yellow azo dye compositions and novel liquid crystal compositions containing the yellow azo dye composition. The yellow azo dye composition, having terminal ester groups, has the general formula: ##STR1## wherein R and R&#39; are alkyl or substituted alkyl wherein the alkyl has at least 4 carbon atoms, aryl, arylalkyl, substituted aryl or substituted arylalkyl. When the yellow azo dichroic dyes are used alone, or in combination with other dyes, as a guest in a host liquid crystal material, they form a novel liquid crystal composition for use in liquid crystal displays. An invention method of improving the black color of dichroic dyes for use in black liquid crystal displays, wherein a dye composition having a mixture of dichroic dyes of various colors and containing at least one yellow azo dichroic dye, as described above, is added to a host liquid crystal material. The mixture of dichroic dyes of various colors must be one which absorbs radiation in the spectral range of about 400-700 nm to produce the substantially black color for use in black liquid crystal displays.

BACKGROUND OF THE INVENTION

This invention relates to yellow azo dye compounds, and moreparticularly, to dichroic liquid crystal compositions containing theyellow azo dyes.

Yellow dyes having improved properties are desirable for variousapplications including the dyeing of textiles and the coloring ofplastics and liquid crystal displays. Among those properties which aredesirable for dyes, especially for dyes used in liquid crystal displays,are photostability of the dye, suitable extinction coefficient of thedye, solubility of the dye in liquid crystal hosts and light absorptionin the desired spectral region. In liquid crystal displays, yellow dyesare especially useful for modifying the color of other dyes or asadditives for black dichroic formulations. The fabrication of blackliquid crystal displays poses a particular problem in that no blackdichroic dyes are known, and black liquid crystal displays are made bymixing dichroic dyes of various colors in proper proportions to obtain amaterial which absorbs light radiation in the spectral range of about400-700 nm resulting substantially in a black color. The dyes used insuch mixtures must have a high order parameter, preferably at least0.75, to ensure satisfactory brightness and contrast ratio. Heretofore,the only available yellow dyes of high order parameter contained Schiffbase linkages within the dye molecule, and the presence of a double ortriple bond in the base unit of the dye (exclusive of substituted groupsand derivatives), leads to chemical and/or photochemical instability ofthe material.

Dye compounds suitable for use in solution with a liquid crystalmaterial and containing azo linkages rather than Schiff base linkages inthe dye molecule, are well known in the prior art. In U.S. Pat. No.4,145,114, there is a disclosure of a substantial number of dyecompounds suitable for use in liquid crystal materials and having an azoor azoxy linkage of the following general formula: ##STR2## or asubstituted derivative of the foregoing formula in which any one of thepositions on any one of the benzene rings is substituted by halogen,methyl, halogen substituted methyl, or methoxy; wherein A is an azo orazoxy linkage group; n₁ and n₂ are each integers in the inclusive range0 to 4; and Y₁ and Y₂ are each one of the following groups: (i)hydrogen; or (ii) OR where R is an alkyl or arylalkyl group. Althoughthe general structure of the dyes of U.S. Pat. No. 4,145,114 avoid theSchiff base likages and thereby eliminate the chemical and/orphotochemical instability disadvantages of materials containing theSchiff base linkages, it is always desirable to improve azo dyes and toimprove liquid crystal displays containing azo dyes.

The present inventors describe in U.S. Pat. No. 4,122,027, certaindichroic dyes of the 4,4'-bis(substituted naphthylazo)azobenzene type,each having three azo bonding groups, namely seven dyes having thegeneral formula: ##STR3## and in U.S. Pat. No. 4,179,395, certaindichroic dyes are described having the general formula: ##STR4## Inpending application Ser. No. 175,003, filed Aug. 4, 1980, the presentinventors describe certain yellow azo dichroic dyes having the generalformula: ##STR5## in which R and R' are acyl. Copending application Ser.No. 175,002, also filed Aug. 4, 1980 teaches certain yellow azo dichroicdyes of the general formula: ##STR6## in which R and R' are acyl. Also,in application Ser. No. 175,004, also filed Aug. 4, 1980, yellow azodyes having the general formula: ##STR7## where R and R' are acyl aredescribed. The above applications and patents are incorporated herein byreference in their entireties.

BRIEF SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, novel yellow azo dyesare provided by aliphatic and aromatic esters of4,4'-bis(2-methyl-4-hydroxyphenylazo)-2,5-dimethylazobenzene. A class ofyellow azo dyes comprising the general formula: ##STR8## wherein R andR' are individually alkyl or substituted alkyl wherein the alkyl has atleast 4 carbon atoms or R₂ is an aryl, arylalkyl, substituted aryl, orsubstituted arylalkyl, has been discovered in accordance with thepresent invention. The acyl groups in the general formula form terminalester groups at the hydroxy positions of the4,4'-bis(2-methyl-4-hydroxyphenylazo)-2,5-dimethylazobenzene.

In accordance with another aspect of the present invention, there isalso provided a liquid crystal composition for use in a liquid crystaldisplay, comprising:

a host liquid crystal material; and

a guest dye dissolved in said host liquid crystal material, said dyebeing of the formula I set forth above.

In another embodiment of the present invention, it has been found thatwhen a liquid crystal composition for use in a liquid crystal displaycomprises a host liquid crystal material and a guest yellow dyedissolved in said host liquid crystal material in conjunction with otherdichroic dyes of various colors which absorb radiation within thespectral range of about 400-700 nm, improved black liquid crystalcompositions can be obtained when said yellow dye is of the formula I.Thus, improved black liquid crystal compositions and a method forimproving the quality of black liquid crystal compositions are possibleby using the yellow dichroic azo dyes having at least one terminal esterlinkage in the paraposition on an ortho-methyl substituted phenylazoazobenzene and one of the benzene rings of the azobenzene has methylgroups substituted in the 2- and 5-positions, in a dye having at leastthree azo linkages. Thus, the invention embraces a method of improvingthe black color of dichroic dyes for use in black liquid crystaldisplays, comprising adding to a host liquid crystal material, a dyecomposition having a mixture of dichroic dyes of various colors inproper proportion to obtain a material which absorbs radiation in thespectral range of about 400-700 nm wherein at least one of the dichroicdyes of various colors is a yellow azo dichroic dye of formula I; andheating the composition to dissolve the dyes in the liquid crystal hostmaterial.

In accordance with the present invention, the chemical and/orphotochemical instability of the prior art has been substantiallyreduced by using the azo linkages in the yellow azo dichroic dyes havingterminal ester groups. Furthermore, a new class of yellow dyes has beensynthesized, and liquid crystal displays of various colors can befabricated by mixing at least one of the new class of yellow azo dyeswith at least one dichroic dye of a different color and dissolving themixture in a host liquid crystal material.

Accordingly, it is the primary object of the present invention toprovide yellow azo dyes having a novel chemical composition.

It is another object of the present invention to provide novel liquidcrystal compositions for use in liquid crystal displays.

Another object of the present invention is to provide yellow azo dyeshaving improved chemical and/or photochemical stability for use inliquid crystal compositions.

Another object of the present invention is to provide improved blackdichroic liquid crystal displays made by mixing dichroic dyes of variouscolors.

Yet another object of the present invention is to provide a method ofimproving the black color of dichroic dyes used in black liquid crystaldisplays.

Still another object of the present invention is to provide a class ofyellow dyes devoid of Schiff base linkages.

These and various other objects, features and advantages of theinvention can be best understood from the following detaileddescription.

DETAILED DESCRIPTION OF THE INVENTION

The yellow azo dyes or guest yellow azo dyes of the present inventionhave been synthesized by a reaction which produces the following generalformula representing the class of yellow azo dyes ##STR9## having arelatively elongated and rigid molecular structure. The yellow dyes havethree azo (--N═N--) linking groups, one of the azo linking groupsjoining an unsubstituted benzene ring to a benzene ring having methylgroups substituted in the 2-position and the 5-position, and the othertwo azo groups joining the foregoing substituted and unsubstitutedbenzene rings at the paraposition to a third and fourth benzene ringhaving a methyl group substituted in the ortho-position (relative to theazo linkage) and an ester group substituted in the paraposition(relative to the azo linkage), so that the parasubstituted ester formsthe terminal portion of the yellow azo dye molecule. The ester endgroups or terminal ester groups normally designated as auxochromes,e.g., electronwithdrawing groups or electron-repelling groups, enhancethe characteristic yellow color of the dyes by enhancing absorption atthe blue end of the spectrum, e.g., 380-430 nm. R and R' in the generalformula for the yellow azo dyes and guest yellow azo dyes, are part ofthe acyl groups which form the ester. In accordance with preferredembodiments of the present invention, R═R'; however, it is within thescope of the present invention to provide dyes of the general formulawherein R and R' comprise different groups, and in fact, R and R' mayrepresent any combination of alkyl, substituted alkyl, aryl, arylalkyl,substituted aryl or substituted arylalkyl as specified in thisinvention.

The acyl groups of the present invention containing R and R' in thegeneral formula I, have the following general formula: ##STR10## Theacyl group is a functional group, and as used herein, is generally thefunctional group of an acid chloride. In various embodiments, the acylgroup of the yellow azo dyes of the present invention, may be anaromatic (aryl) group or a substituted aromatic group or mixturesthereof. However, the acyl group may also be an aliphatic (alkyl),substituted aliphatic, arylalkyl or substituted arylalkyl or mixturesthereof. Thus, in accordance with the present invention, R₂ in theformula representing the acyl groups, may be aryl, arylalkyl,substituted aryl, substituted arylalkyl, alkyl having at least 4 carbonatoms or substituted alkyl having at least 4 carbon atoms, or mixturesof the foregoing.

The dyes of the present invention may be synthesized by any suitablemanner. In one preferred method of synthesis, 2,5-dimethyl-azodianilineis tetrazotized using sodium nitrite and dilute hydrochloric acid. Thetetrazonium solution is then coupled with meta-cresol in a basicsolution to give4,4'-bis(2-methyl-4-hydroxyphenylazo)-2,5-dimethylazobenzene. Thisproduct is then reacted with the appropriate acid chloride in a suitablesolvent, for example, pyridine. The mixture is heated for an appropriatetime and at a suitable temperature to complete the reaction, e.g., about1-4 hours at about 80° C.-120° C. Upon cooling, the yellow dyeprecipitates and is collected. In certain instances, hexane or othersuitable aliphatic hydrocarbons may be used to promote precipitation. Inthe foregoing synthesis, for every mole of 2,5-dimethyl-azodianiline,about two moles of hydrochloric acid and about two moles of sodiumnitrite were used. Two moles of the coupling agent (m-cresol) was usedfor every mole of 2,5-dimethyl-azodianiline. If both acyl groups on thedye molecule are the same, a slight molar excess of the appropriate acidchloride, such as benzoyl chloride, may be used. If the acyl groupsdiffer on the same molecule, the acylation is carried out in twoseparate steps. The first acylation is carried out with about one moleof the appropriate acid chloride per mole of 2,5-dimethyl-azodianilinederivative in a basic solution such as an organic base (pyridine), andthe solution is heated at a suitable temperature until the reaction iscomplete, e.g., it is heated for three hours at 110° C. A secondacylation is carried out with about one mole of the appropriate acidchloride per mole of the monoester of 2,5-dimethylazodianilinederivative in a basic solution such as an organic base (pyridine), andthe solution is heated at a suitable temperature until the reaction iscomplete, e.g., it is heated for three hours at 110° C. Thereafter, thesolution is cooled. The yellow dye precipitates and is collected. Aprecipitation agent, such as n-hexane, may be added to promote theprecipitation of the dye from the solution.

In preferred embodiments, the acyl group represented by the formula:##STR11## is an aromatic or substituted aromatic group designated hereinas an aryl or substituted aryl group. As used herein, aryl generallyrefers to a phenyl group or a naphthyl group. Examples of several acylgroups which form the ester linkage with the4,4'-bis(2-methyl-4-hydroxyphenylazo)-2,5-dimethylazobenzeneintermediate formed from the tetrazonium solution and the m-cresol asdiscussed above, are shown in Table 1 below where the illustrated acylgroup represents any combination of R and R' in the general formula ofthis invention but preferably where R═R'.

                  TABLE 1                                                         ______________________________________                                        EXAMPLE OF YELLOW OR YELLOW-ORANGE                                            AZO DYES HAVING ACYL GROUPS                                                        ACYL Group                                                               Dye  (RCO and R'CO) of GENERAL                                                                         CORRESPONDING                                        NO.  FORMULA             ACID CHLORIDE                                        ______________________________________                                              ##STR12##          benzoyl chloride                                           ##STR13##          p-anisoyl chloride                                         ##STR14##          4-pentoxybenzoyl chloride                                  ##STR15##          4-heptoxybenzoyl chloride                                  ##STR16##          3,5-dimethoxyben- zoyl chloride                            ##STR17##          4-pentylbenzoyl chloride                                   ##STR18##          4-heptylbenzoyl chloride                                   ##STR19##          4-cyanobenzoyl chloride                                    ##STR20##          p-nitrobenzoyl chloride                              ______________________________________                                    

Other aromatic (aryl) acyl groups can be used in the general formula andcan be prepared from the corresponding acid chloride. Other aryl groupsinclude benzene (phenyl) and naphthalene (naphthyl) and the halogen,nitro, cyano, amino, alkyl, aryloxy, biaryl, alkoxy derivativessubstituted upon the aryl group, mixtures and combinations of theforegoing derivatives substituted upon the aryl group, and the like.Other exemplary aromatic acyl groups preferred in accordance with thepresent invention include para-n-decylbenzoyl, p-ethoxybenzoyl,p-butoxybenzoyl, p-hexyloxybenzoyl, p-N,N-dimethylaminobenzoyl,p-N,N-diethylaminobenzoyl, p-N,N-dibutylaminobenzoyl, p-hydroxybenzoyl,p-chlorobenzoyl, p-bromobenzoyl, p-benzoyloxybenzoyl, p-phenoxybenzoyl,p-toluyl, p-ethylbenzoyl, and p-phenylbenzoyl. Naturally, it is withinthe purview of one skilled in the art to add a considerable number ofacyl groups to the foregoing list. As explained above, dyes of thegeneral formula with different R and R' groups can be synthesized by thereaction of one mole of the4,4'-bis(2-methyl-4-hydroxyphenylazo)2,5-dimethylazobenzene intermediatewith one mole of the appropriate acyl chloride to provide a reactionproduct having one acyl group thereon, followed by the reaction of onemole of the reaction product having one acyl group thereon with one moleof another appropriate acyl chloride to form the yellow azo dyecomposition having two different acyl groups thereon.

For example, in a first reaction, about 1.0 mole of benzoyl chloride permole of the 4,4'-bis(2-methyl-4-hydroxyphenylazo)-2,5-dimethylazobenzeneintermediate is heated at about 110° C. for about three hours to form anester as follows: ##STR21## In a second reaction, about 1.0 mole ofp-anisoyl chloride per mole of the ester of formula (II) above is heatedat about 100° C. for about three hours under the reaction conditionsdiscussed above to form an ester (III) as follows: ##STR22## The yellowazo dye (III) is 4-(2-methyl-4-benzoyloxyphenylazo)-4'-(2-methyl-4-anisoyloxy phenylazo)-2,5-dimethylazobenzene.

The acyl groups of the present invention may also be alkyl orsubstituted alkyl acyl groups wherein the alkyl or substituted alkyl hasat least 4 carbon atoms. The acyl groups of the present invention canalso be arylalkyl or substituted arylalkyl acyl groups. Thus, inaccordance with the general formula of this invention, R and R' may be apart of the foregoing enumerated acyl groups of the formula: ##STR23##wherein R₂ is an alkyl or substituted alkyl and the alkyl has at least 4carbon atoms, or R₂ can be arylalkyl or substituted arylalkyl. Thesubstituted groups may be any of the conventional derivatives well knownin the art including the halogens (e.g., chlorine and bromine), hydroxy,nitro, cyano, amino, alkyl, aryloxy, alkoxy, and the like, and variouscombinations and permutations of the foregoing derivatives. In preferredembodiments, the foregoing acyl derivatives can be obtained by using thecorresponding acid chloride in the synthesis described above. Inaccordance with the present invention, the acyl groups on the yellow azodye molecule can be the same or different groups including differentalkyl or substituted alkyl or different arylalkyl or substitutedarylalkyl groups and can also comprise the foregoing groups mixed witharomatic acyl groups.

When the invention has alkyl or substituted alkyl acyl groups, inpreferred embodiments, the alkyl or substituted alkyl has about four toabout 20 carbon atoms. Preferred alkyl groups include n-butyl,iso-butyl, n-amyl, n-hexyl, n-heptyl, n-octyl, n-nonyl, n-decyl andstearyl and the like and the halogen (e.g., chlorine and bromine),nitro, amino, hydroxy and cyano derivatives thereof.

Specific examples of alkyl acid chlorides which may be used in areaction with the4,4'-bis(2-methyl-4-hydroxy-phenylazo)-2,5-dimethylazobenzeneintermediate described above in the appropriate molar quantities includen-butyryl chloride, iso-butyric acid chloride, n-valeryl chloride,iso-valeryl chloride, n-caprylic acid chloride (octanoyl chloride),pelargonic acid chloride (pelargonyl chloride or nonanoyl chloride),n-caproic acid chloride (n-caproyl chloride), n-capric acid chloride,stearic acid chloride (stearyl chloride), palmityl chloride, and thelike.

Examples of the above yellow azo dyes having alkyl acyl groups are shownin Table 2 below where the acyl group and the corresponding acidchloride are illustrated:

                  TABLE 2                                                         ______________________________________                                        EXAMPLES OF YELLOW AZO DYES HAVING                                            ALKYL ACYL GROUPS                                                                   ACYL GROUP (RCO and                                                     DYE   R'CO ) OF GENERAL  CORRESPONDING                                        NO.   FORMULA            ACID CHLORIDES                                       ______________________________________                                               ##STR24##         n-butyryl chloride                                          ##STR25##         iso-butyric acid chloride                                   ##STR26##         n-valeryl chloride                                          ##STR27##         iso-valeryl chloride                                        ##STR28##         octanoyl chloride                                           ##STR29##         n-caproyl chloride                                          ##STR30##         pelargonyl chloride                                         ##STR31##         n-capric acid chloride                                      ##STR32##         stearyl chloride                                     10.                                                                                  ##STR33##         palmityl chloride                                    ______________________________________                                    

To prepare the yellow azo dyes of this invention having alkyl acylgroups, substituted alkyl acyl groups, arylalkyl acyl groups, orsubstituted arylalkyl acyl groups, the corresponding acid chloride (acylchloride) is reacted with the4,4'-bis(2-methyl-4-hydroxyphenylazo)-2,5-dimethylazobenzeneintermediate in the same manner described above for the preparation ofthe aromatic acyl compounds. Thus, 2,5-dimethyl-azodianiline istetrazotized to form an intermediate tetrazonium chloride solution whichis coupled with m-cresol in a basic solution to yield the4,4'-bis-(2-methyl4-hydroxyphenylazo)-2,5-dimethyl-azobenzeneintermediate. This product is then reacted with the appropriatealiphatic (alkyl) acid chloride or arylaliphatic (arylalkyl) acidchloride as discussed above. A slight molar excess of the appropriatealiphatic acid chloride is added to the4,4'-bis(2-methyl-4-hydroxyphenylazo)-2,5-dimethyl-azobenzeneintermediate to produce the yellow azo dye product. For example,slightly over 2 moles of stearyl chloride is added to the4,4'-bis-(2-methyl-4-hydroxyphenylazo)-2,5-dimethyl-azobenzene inpyridine solvent, and the mixture is heated for about three hours atabout 110° C. to form the product. Upon cooling, the yellow azo dyeprecipitates and may be filtered from the solution. Conventionalprecipitating promoters, such as n-hexane, can be added to promoteprecipitation.

The yellow azo dyes prepared in accordance with this invention can beused in any conventional manner. For example, they can be used alone orin combination with other dyes, modifiers or adjuvants to dye textilesor to color plastic materials. They can be used alone or in combinationwith other dyes, modifiers or adjuvants to color liquid crystaldisplays. In preferred embodiments, the yellow azo dyes are used withliquid crystal host materials well known in the art to improve the colorof the liquid crystal composition for use in a liquid crystal display.When the yellow azo dyes of this invention are used with a liquidcrystal host, they are called guest dyes, and they are dissolved in thehost liquid crystal material, that is, the guest yellow azo dye issoluble in the host liquid crystal material. Many of the yellow azo dyesrequire mild heat, e.g., above ambient to 110° C. to promote theirdissolution in the host liquid crystal.

Any liquid crystal host material may be used in accordance with thepresent invention. In those embodiments of the present invention whereinthe yellow tris-azo dichroic dye material is used as a guest dye in ahost liquid crystal material, it is not critical which of the many wellknown liquid crystal materials or combinations thereof may be used aslong as the yellow azo dye materials of the present invention aresoluble in the host liquid crystal material and as long as the yellowazo dye or dyes of the present invention are compatible with or have noadverse effect upon the host liquid crystal material. In preferredliquid crystal compositions of the present invention, the host liquidcrystal material is a nematic liquid crystal of positive or negativedielectric anisotropy, a cholesteric liquid crystal of positive ornegative dielectric anisotropy and mixtures thereof, that is, mixturesof nematic liquid crystals and cholesteric liquid crystals modified withother optically active compounds.

Any conventional liquid crystal host material can be used with theyellow tris-azo dyes of the present invention. The liquid crystal hostscan be selected by one skilled in the art depending upon theelectro-optic effect to be utilized. Nematic liquid crystals include thebiphenyl liquid crystals such as E-7 and E-8 from BDH, Ltd.

Biphenyl liquid crystal E-7 is a eutectic of the following generalformula: ##STR34## where R is a mixture of C₅ H₁₁, C₇ H₁₅, C₈ H₁₇ O and##STR35## Other nematic liquid crystals include esters available from E.Merck Associates and HoffmanLaRoche Co., phenylcyclohexanes and azoxymixtures available from E. Merck Associates, alkoxybenzylidene anilssuch as those having the structural formula: ##STR36## wherein OR is analkoxy radical preferably having from 1 to 7 carbon atoms or an acyloxyradical having from 2 to 7 carbon atoms,p-anisylidene-p'-n-butylaniline, p-anisylidene-p'-aminophenyl-butyrate,p-(p'-methoxyphenylazoxy)butylbenzene,p-(p'-ethoxyphenylazo)phenylheptanoate, p-n-hexylbenzoicacid-p'-n'-hexyloxy-phenyl ester, and other liquid crystal materials,such as the esters disclosed in U.S. Pat. No. 3,984,344,p-n-hexylbenzylidene-p'-amino-benzonitrile,p-capryloxybenzylidene-p'-aminobenzonitrile,p-cyanophenyl-p'-n-heptylbenzoate,p-cyanobenzylidine-p'-n-butoxyaniline,p-cyanobenzylidene-p'-aminophenylvalerate, p-azoxyanisole,butyl-p-(p'-ethoxyphenoxycarbonyl)phenylcarbonate,p(p'-ethoxyphenylazo)phenylheptanoate, and the like. Another class ofliquid crystal materials have the general formula: ##STR37## wherein Ris an alkyl or alkoxy group and m=0 or 1, at least one of said compoundsbeing cyanobiphenyl wherein m=0. Generally, nematic liquid crystals fallwithin the class of chemical compounds having the general formula:##STR38## wherein X is an azomethine linkage of the Schiff base class, Xis an ester, vinylene, acetylene, azo or azoxy, or X is merely a singlebond connecting the two benzene rings; and A and B are C_(n) H_(2n+1)(alkyl group); C_(n) H_(2n+1) O (alkoxy group); or C_(n) H_(2n+1) COO(ester group). Cholesteric liquid crystals can be pure compounds such asderivatives of cholesterol or noncholesteric materials. These arecharacterized by optical activity arising from the absence of molecularsymmetry.

In accordance with the present invention, it is not critical how much ofthe dye is present in the host liquid crystal as long as the dye issoluble therein. In preferred embodiments, about 0.05% by weight toabout 1.0% by weight, and more preferably about 0.1% by weight to about0.5% by weight, (based upon the weight of the liquid crystal hostmaterial) of the dye of this invention or mixtures of dyes of thisinvention are present in the host liquid crystal. One skilled in the artcan adjust the amount of yellow tris-azo dye as desired and candetermine the amount required for maximum absorption at a particularwavelength. The upper limit of the amount of dye varies with thesolubility of the particular dye in the host liquid crystal. The amountof guest dichroic yellow tris-azo dye of the present invention in thehost liquid crystal material is that amount up to the limit of maximumsolubility in the host of yellow tris-azo dye required to color, tint orshade the host liquid crystal, required to add to the yellow color ofthe host liquid crystal material, or required to contribute to the colorof a mixture of dyes used in the host liquid crystal material, e.g., theuse of the yellow dye or dyes of this invention with dyes of othercolors to improve the "blackness" of guest dyes in the host liquidcrystal. The guest yellow azo dichroic dyes of this invention areparticularly useful in black liquid crystal displays because improvedblack liquid crystal materials can be obtained by mixing dichroic dyesof various colors in proper proportions to obtain a material whichabsorbs radiation in the spectral range of about 400-700 nm, and theyellow azo dyes of this invention substantially improve the absorptionin the 400 nm region (the blue region) of such dye mixtures. Thus, inaccordance with the present invention, an improved method of makingblack dichroic dyes or black dichroic liquid crystal displays isachieved by providing a host liquid crystal material and mixingtherewith dichroic dyes of various colors in suitable proportions toobtain a material which absorbs radiation in the spectral range of about400- 700 nm wherein the dichroic dyes of various colors include a dyehaving the general formula I which has been set out hereinbefore. Theappropriate mixture of dichroic dyes of various colors in suitableproportions to obtain a material which absorbs radiation in the spectralrange of about 400-700 nm, and which approaches a substantially blackappearance in color, is within the purview of one skilled in the art,and the improvement of the present invention is directed to the use ofone or more of the yellow tris-azo dyes having the designated acyl orester groups to improve the "blackness" of the dichroic dye mixture madefrom various colors. For example, the yellow tris-azo dyes of thisinvention, when mixed with appropriate blue dye in a liquid crystal,produce an improved greenish-black display. The yellow tris-azo dyes ofthis invention used in such mixtures have substantially high orderparameters (S) generally at least 0.75 and above, to ensure satisfactorybrightness and contrast ratio.

Two types of liquid crystal displays having two different liquid crystalhosts were prepared with the dyes of the present invention. In one ofthe types of liquid crystal display, the yellow azo dyes were dissolvedat a concentration of about 0.5% to about 1% (by weight of the liquidcrystal host material) in a nematic liquid crystal such as E-7 (from BDHCo.), ROTN-404 (from Hoffman LaRoche) or Licristal-1132 (from E. MerckAssociates). The resulting solution was sandwiched between conductiveglass plates in test cells whose surfaces are in contact with thesolution, the surfaces having been previously rubbed unidirectionally.When illuminated with polarized light, the display switches from yellowin the absence of an applied voltage to substantially colorless in thepresence of a voltage. In the other type of liquid crystal display usedto demonstrate the present invention, the liquid crystal host was acholesteric liquid crystal prepared by the addition of an opticallyactive compound, such as CB-15 (from BDH Co.) to one of theabove-mentioned nematic liquid crystal materials. The concentration ofthe optically active compound was varied from about 1 % to about 10%.The yellow tris-azo dyes of this invention were dissolved at aconcentration of about 0.5 to about 1.0% in the cholesteric host. Uponapplication of a voltage, the yellow display became substantiallycolorless.

Examples of specific commercial nematic liquid crystals include BDHChemical's E-7 biphenyl type materials, whose temperature range is -10°to 60° C. with a dielectric anisotropy of 11, and E-8 with a temperaturerange of about -10° to 70° C. with a dielectric anisotropy of 13; E.Merck Associates' phenylcyclohexane type materials Licristal 1132 havinga temperature range of -6° to 70° C. with a dielectric anisotropy of 10and Licristal 1221 having a temperature range of -10° to 90° C. withdielectric anisotropy of 8; Hoffman-LaRoche's ROTN-103 an ester typehaving a temperature range from about -10° to 80° C. with dielectricanisotropy of 26, ROTN-200, a Schiff base type having a temperaturerange of about -15° to 65° C. with a dielectric anisotropy of 18, andROTN-404, a biphenyl-pyrimidine type having a temperature range fromabout -10° to 105° C. and a dielectric anisotropy of 21.

The following examples further illustrate the practice of the invention,and they are meant to be exemplary only and are not to be construed asto limiting the invention in any way.

EXAMPLE 1 Preparation of Tris-Azo Dye

The tetrazotization of 2,5-dimethyl-azodianiline was carried out bydissolving 0.02 moles (4.8 grams) of the azodianiline in about 60 mls.of dilute hydrochloric acid. The temperature of the resulting mixturewas maintained at about 5° C. while adding about 3 grams (about 0.044moles) of sodium nitrite in about 10 mls. of water. The resultingtetrazonium solution was then coupled with about 0.04 M of m-cresol,previously dissolved in dilute potassium carbonate (10%) and cooled to atemperature of about 5° C. The coupling reaction product was4,4'-bis(2-methyl-4-hydroxyphenylazo)-2,5-dimethyl-azobenzene. Thisproduct was then reacted with about 0.04 M of 4-pentyloxybenzoylchloride dissolved in pyridine. The solution was heated for about threehours at about 110° C. Upon cooling a yellow dye precipitated and wascollected by filtration. The yellow dye is4,4'-bis[2-methyl-4-(4-pentyloxybenzoxy)phenylazo]-2,5-dimethyl-azobenzene.The dye was dissolved in Merck 1291, a phenylcyclohexane liquid crystalcommercially available and had a solubility on the order of 1%, anabsorption maximum at 425 nm and an order parameter S of 0.78.

EXAMPLE 2

The yellow azo dye prepared in Example 1 above (1.0% by weight basedupon the weight of the liquid crystal host material) is added to aliquid crystal host mixture of ROTN-404, a biphenyl-pyrimidine ofpositive dielectric anisotropy having a temperature range of -10° C. to105° C. The formulation was heated to dissolve the dye in the hostmaterial and then filtered. The resulting formulation was then placed ina test cell having a spacing between the cell substrates of about 18microns and treated to give parallel alignment of liquid crystal uponthe surface. When illuminated with polarized light, the display wasyellow in the absence of any applied voltage and is substantiallycolorless in the presence of applied voltage.

EXAMPLE 3

Example 2 was repeated except that the liquid crystal host material wasE-7.

EXAMPLE 4

Example 2 is repeated except that the liquid crystal host material isLicristal 1132.

EXAMPLE 5

Examples 2, 3 and 4 are repeated except that the liquid crystal hostmaterial contains about 5 wt.% of CB-15 optically active chiral dopantfrom BDH Ltd. CB-15 is 4-cyano-4'-(2-methylbutyl) biphenyl and convertsthe nematic liquid crystals into cholesteric liquid crystals.

EXAMPLE 6

Example 2 is repeated employing 0.5 wt.% of the yellow azo dye preparedin Example 1 and 0.5% of the yellow dye4,4'-bis-[2-methyl-4-(4-pentoxybenzoxy)-phenylazo]-2-methyl-5-methoxyazobenzene.

Utility of the yellow tris-azo dyes of this invention has beendemonstrated. When used alone the dyes can be used to provide yellowdisplays or they can be used as additives in conjunction with other dyesto provide improved black dyes for black dichroic displays when usedwith liquid crystals. The dyes of the present invention can also be usedas additives with other dyes to obtain varied or unusual color effectsin liquid crystal displays. Furthermore, the yellow tris-azo dyes of thepresent invention have been shown to be useful with a wide variety ofliquid crystal compositions and are compatible with known liquidcrystals, such as Schiff base, esters, azoxy, biphenyl,phenylcyclohexane, biphenyl/pyrimidinetype liquid crystals and the like.The novel yellow tris-azo dyes of the present invention can also be usedfor the dyeing of textiles, coloring of plastics or the coloring of anyother media or substrates conventionally colored by azo dyes.

While other modifications of the invention and variations thereof whichmay be employed within the scope of the invention have not beendescribed, the invention is intended to include such modifications asmay be embraced within the following claims.

What is claimed is:
 1. A liquid crystal composition for use in a liquidcrystal display, comprising:a host liquid crystal material; and a guestdichroic dye dissolved in said host liquid crystal material, saiddichroic dye having the general formula: ##STR39## wherein R and R' areindividually alkyl or halogen, nitro, amino, hydroxy or cyanosubstituted alkyl and the alkyl has from about 4 to about 20 carbonatoms, or phenyl; phenylalkyl; halogen, nitro, cyano, hydroxy,dialkylamino, amino, alkyl, phenyloxy, phenyl, or alkoxy parasubstitutedphenyl or phenylalkyl; or 3,5-dialkoxyphenyl radical.
 2. The compositionof claim 1, wherein R═R'.
 3. The composition of claim 1, wherein R andR' are selected from the group consisting of n-butyl, iso-butyl, n-amyl,n-hexyl, n-heptyl and stearyl and the halogen, nitro, amino and cyanoderivatives thereof.
 4. The composition of claim 1, wherein R and R' arean alkyl having from 4 to about 20 carbon atoms or the halogen, nitro,amino, or cyano derivatives thereof.
 5. The composition of claim 1,wherein the phenyl or phenylalkyl parasubstituent is selected from thegroup consisting of halogen, hydroxy, nitro, cyano, amino, alkyl,phenyloxy and alkoxy.
 6. The composition of claim 1, wherein R and R'are p-pentoxyphenyl.
 7. The composition of claim 1, wherein about 0.05%by weight to about 1.0% by weight (based upon the weight of the liquidcrystal) of the guest dichroic dye is dissolved in the host liquidcrystal.
 8. The composition of claim 1, wherein the host materialcomprises a nematic liquid crystal or a cholesteric liquid crystal.
 9. Amethod of improving the black color of dichroic dyes for use in blackliquid crystal displays, comprising the steps of: adding to a hostliquid crystal material a dye composition having a mixture of dichroicdyes of various colors in proper proportion to obtain a material whichabsorbs radiation in the spectral range of about 400-700 nm; selectingat least one of the dichroic dyes of various colors to be a yellow azodichroic dye having the general formula: ##STR40## wherein R and R' areindividually alkyl or halogen, nitro, amino, hydroxy or cyanosubstituted alkyl and the alkyl has from about 4 to about 20 carbonatoms, phenyl; phenylalkyl; halogen, nitro, cyano, hydroxy,dialkylamino, amino, alkyl, phenyloxy, phenyl, or alkoxy parasubstitutedphenyl or phenylalkyl; or 3,5-dialkoxyphenyl radical; and heating thecomposition to dissolve the dyes in the liquid crystal host material.10. The method of claim 9, wherein about 0.05% to about 1.0% of theyellow azo dichroic dye is added to the host liquid crystal material andthe dichroic dyes of various colors.
 11. The method of claim 9, whereinR═R'.
 12. The method of claim 9, wherein R and R' are alkyl having from4 to about 20 carbon atoms or the halogen, nitro, amino or cyanoderivatives thereof.
 13. The method of claim 9, wherein the mixture isheated to about 60° C. to about 105° C. to dissolve the dichroic dyes inthe host material.
 14. The method of claim 9, wherein the phenyl orphenylalkyl parasubstituent is selected from the group consisting ofhalogen, hydroxy, nitro, cyano, amino, alkyl, phenyloxy and alkoxy. 15.An improved black dichroic liquid crystal display compositioncomprising:a host liquid crystal material; and a dye compositiondissolved in said host, the dye composition having a mixture of dichroicdyes of various colors in proper proportion to obtain a material whichabsorbs radiation in the spectral range of about 400-700 nm wherein atleast one of the dichroic dyes of various colors is a yellowazo-dichroic dye having the general formula: ##STR41## wherein R and R'are individually alkyl or substituted alkyl, and the alkyl has halogen,nitro, amino, hydroxy, or cyano substituted alkyl, and the alkyl hasfrom about 4 to about 20 carbon atoms, phenyl; phenylalkyl; halogen,nitro, cyano, hydroxy, dialkylamino, amino, alkyl, phenyloxy, phenyl, oralkoxy parasubstituted phenyl or phenylalkyl; or 3,5-dialkoxyphenylradical.
 16. The black dichroic liquid crystal display composition ofclaim 15, comprising about 0.05% to about 1.0% by weight (based upon theweight of the liquid crystal) of the yellow azo dichroic dye.
 17. Theblack dichroic liquid crystal display composition of claim 15, whereinR═R'.
 18. The black dichroic liquid crystal display composition of claim15, wherein R and R' are alkyl having from 4 to about 20 carbon atoms orthe halogen, nitro, amino or cyano derivatives thereof.
 19. The blackdichroic liquid crystal display composition of claim 15, wherein thephenyl or phenylalkyl parasubstituent is selected from the groupconsisting of halogen, hydroxy, nitro, cyano, amino, alkyl, phenyloxyand alkoxy.
 20. The dye composition of claim 15, wherein R and R' are4-pentoxyphenyl.
 21. The composition of claim 15, wherein the hostmaterial comprises a nematic liquid crystal or a cholesteric liquidcrystal.